I'm fairly sure that when Bio_Hazard1282_rPi3 does not match any existing user name talks about unused hex codes, he is talking about addresses on the calculator's memory that correspond to commands, tokens, or pointers. It is a little complex to describe, but every process that the calculator runs has an address in memory. Whether it be typing an equation in, or generating a random variable, or something else, it has a location (a Hex Code). There are many, many of these locations that are unused just due to the design of the calculator. For example, one page of ROM storage is 65535 bits if I remember correctly, and there are multiple pages of ROM just for the operating system.
What I believe you are asking for is if there are any hidden commands or characters that aren't used at all. And the answer is, yes, there are characters that are "not used" at least in the sense that they aren't accessible to type in normally. You can see all the TI font characters on the 83lgfont page. As for commands or modes, I don't know enough about it to be certain